Electrode for oxygen evolution in industrial electrochemical processes

ABSTRACT

An electrode for electrolytic processes, in particular to an anode suitable for oxygen evolution having a valve metal substrate, a catalytic layer, a protection layer consisting of oxides of valve metals interposed between the substrate and the catalytic layer and an outer coating of oxides of valve metals. The electrode is particularly suitable for processes of cathodic electrodeposition of chromium from an aqueous solution containing Cr (III).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/439,908filed on Apr. 30, 2015, which is a 371 of PCT/EP2013/075055, filed Nov.29, 2013, which claims the benefit of priority from Italian PatentApplication Serial No. MI2012A002035, filed Nov. 29, 2012, the contentsof each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an electrode for electrolytic processes, inparticular to an anode suitable for oxygen evolution in an industrialelectrolytic process and to a method of manufacturing thereof.

BACKGROUND OF THE INVENTION

The invention relates to an electrode for electrolytic processes, inparticular to an anode suitable for oxygen evolution in an industrialelectrolysis process. Anodes for oxygen evolution are widely used invarious electrolytic applications, several of which fall within thefield of cathodic metal electrodeposition (electrometallurgy) and covera wide range in terms of applied current density, which can be verysmall (for instance a few hundred A/m², such as in metal electrowinningprocesses) or very high (such as the case of some galvanicelectrodeposition applications, which can operate above 10 kA/m²,referred to the anodic surface); another field of application of anodesfor oxygen evolution is cathodic protection with impressed current.

In the field of electrometallurgy, with particular reference to metalelectrowinning, the use of lead-based anodes is traditionally widespreadand still valuable for some applications although presenting a ratherhigh oxygen evolution overpotential besides entailing the well knownenvironmental and human health concerns associated with the use of thismaterial. More recently—especially for high current densityapplications, which benefit the most from energy savings associated witha more reduced oxygen evolution potential—electrodes for anodicevolution of oxygen obtained from valve metals substrates, for exampletitanium and alloys thereof, coated with catalytic compositions based onnoble metals or oxides thereof have been introduced on the market.

It should also be considered that the operating lifetime of anodes basedon metal or metal oxide-coated valve metal substrates is greatly reducedin the presence of particularly aggressive contaminants, capable ofestablishing accelerated phenomena of corrosion or pollution of theanode surface. There are also known, in fact, anodes comprising asubstrate coated with a catalytic composition and provided with an outercoating of valve metal oxides for the sake of improving durability. Inthe latter case, however, the presence of the outer valve metal oxidelayer, if too thick, increases the potential to unacceptable values.

It has thus been evidenced the need for providing anodes for oxygenevolution characterised by adequate oxygen overpotential and durationovercoming the drawbacks of prior art electrodes in process conditionsinvolving the presence of additives, such as in decorative chromeplating with trivalent chromium.

SUMMARY OF THE INVENTION

Various aspects of the invention are set out in the accompanying claims.

Under one aspect, the invention relates to an electrode suitable foroxygen evolution in electrolytic processes comprising a valve metalsubstrate, a catalytic layer, a protective layer consisting of valvemetals oxides interposed between the substrate and the catalytic layerand an external layer of valve metal oxides, said catalytic layercomprising oxides of iridium, of tin and of at least one doping elementM selected between bismuth and tantalum, the molar ratio of Ir:(Ir+Sn)ranging from 0.25 to 0.55 and the molar ratio M:(Ir+Sn+M) ranging from0.02 to 0.15.

In one embodiment, the molar ratio M:(Ir+Sn+M) of the catalytic layer ofthe electrode according to the invention ranges from 0.05 to 0.12.

In a further embodiment, the molar concentration of iridium in thecatalytic layer ranges between 40 and 50% with respect to the sum ofiridium and tin; the inventors have found that in this composition rangethe element doping is particularly effective in allowing the formationof crystallites of reduced dimensions and high catalytic activity, forexample having a size below 5 nm. The inventors have also observed thatwhen the catalytic layer has a composition and a crystallite size asdescribed, the deposition of an additional external layer of valve metalhaving a barrier function leads to a more regular and homogeneousoverall morphology, so that the increase of potential due to theaddition of such external layers atop the catalytic layer is muchreduced.

In one embodiment, the protective layer interposed between the catalyticlayer and the valve metal substrate comprises a valve metal oxidecapable of forming a thin film impervious to electrolytes, for exampleselected between titanium oxide, tantalum oxide or mixtures of the two.This has the advantage of further protecting the underlying substratemade of titanium or other valve metal from the attack of aggressiveelectrolytes, for example in processes such as those typical of metalplating.

In one embodiment, the electrode is obtained on a substrate of titanium,optionally alloyed; compared to the other valve metals, titanium ischaracterised by a reduced cost coupled to a good corrosion resistance.Furthermore, titanium has a good machinability, allowing its use forobtaining substrates of various geometry, for example in form of planarsheet, punched sheet, expanded metal sheet or mesh, according to theneeds of different applications.

In a further embodiment, the electrode has a specific loading of valvemetal oxides in the external layer ranging from 2 to 25 g/m². Theinventors have surprisingly found that such barrier layer applied bythermal decomposition atop a catalytic layer as hereinbefore describedproduces a beneficial increase in the duration of electrodes used foranodic oxygen evolution, particularly in the range from 2 to 7 g/m², anda lesser increase in potential compared to that observable upon addingthe same to catalytic layers of the prior art.

In a further embodiment, the electrode of the invention has a specificloading of valve metal oxides in the external layer ranging from 9 to 25g/m². The inventors have surprisingly observed that even with theseincreased amounts of valve metal oxides in the outer layer, the anodicpotential is still better than the one typical of the addition tocatalytic layers of the prior art and additionally that the layer servesas an effective barrier against the diffusion of compounds and ionspresent in the electrolyte to the catalytic layer. These combinedfeatures, namely a lower anodic potential and a substantial decrease ofdiffusion, are for instance very important for decorative chromeplating, since a potential reduction even of a mere 50 mV at 1000 A/m²coupled to a lesser diffusion of Cr (III) ions decreases the kinetics ofthe parasitic anodic reaction of Cr (III) oxidation to Cr (VI), whichcan seriously impair the quality of the cathodic deposit of chromiummetal. In the prior art, Cr (VI) production due to parasitic reaction isusually compensated by supplying additives requiring a periodic purgingof the bath and their subsequent restoring with fresh solution.

In one embodiment, the electrode of the invention is provided with anexternal layer of valve metal oxides made of one component selectedbetween titanium oxide and tantalum oxide.

Under another aspect, the invention relates to a method formanufacturing an electrode suitable for use as oxygen-evolving anode inelectrolytic processes comprising the application in one or more coatsof a solution containing precursors of iridium, tin and said at leastone doping element M to a valve metal substrate and the subsequentdecomposition of said solution by heat treatment in air at a temperatureof 480 to 530° C., with formation of said catalytic coating and withformation of said external layer by application and subsequent thermaldecomposition of a solution containing a precursor of titanium ortantalum.

Prior to said step of catalytic coating application, the substrate maybe provided with a protective layer of valve metal oxides applied byprocedures such as flame or plasma spraying, prolonged heat treatment inan air atmosphere, thermal decomposition of a solution containingcompounds of valve metals such as titanium or tantalum, or other.

Under another aspect, the invention relates to a process of cathodicelectrodeposition of metals from an aqueous solution wherein the anodichalf-reaction is an oxygen evolution reaction carried out on the surfaceof an electrode as hereinbefore described.

Under a further aspect, the invention relates to a process of cathodicelectrodeposition of chromium from an aqueous solution containing Cr(III).

The following examples are included to demonstrate particularembodiments of the invention, whose practicability has been largelyverified in the claimed range of values. It should be appreciated bythose of skill in the art that the compositions and techniques disclosedin the examples which follow represent compositions and techniquesdiscovered by the inventors to function well in the practice of theinvention; however, those of skill in the art should, in light of thepresent disclosure, appreciate that many changes can be made in thespecific embodiments which are disclosed and still obtain a like orsimilar result without departing from the scope of the invention.

EXAMPLE 1

A titanium sheet grade 1 of 200 m×200 m×3 mm size was degreased withacetone in a ultrasonic bath for 10 minutes and subjected first tosandblasting with corundum grit until obtaining a value of superficialroughness Rz of 40 to 45 μm, then to annealing for 2 hours at 570° C.,then to an etching in 27% by weight H2SO4 at a temperature of 85° C. for105 minutes, checking that the resulting weight loss was comprisedbetween 180 and 250 g/m².

After drying, a protective layer based on titanium and tantalum oxidesat a 80:20 molar ratio was applied to the sheet, with an overall loadingof 0.6 g/m² referred to the metals (equivalent to 0.87 g/m² referred tothe oxides). The application of the protective layer was carried out bypainting in three coats of a precursor solution—obtained by addition ofan aqueous TaCI₅ solution, acidified with HCl, to an aqueous solution ofTiCI₄—and subsequent thermal decomposition at 515° C.

A 1.65 M solution of Sn hydroxyacetochloride complex (hereinafter:SnHAC) was prepared according to the procedure disclosed in WO2005/014885.

A 0.9 M solution of Ir hydroxyacetochloride complex (hereinafter: IrHAC)was prepared by dissolving IrCl₃ in 10% vol. aqueous acetic acid,evaporating the solvent, adding 10% aqueous acetic acid with subsequentsolvent evaporation twice more, finally dissolving the product in 10%aqueous acetic acid again to obtain the specified concentration.

A precursor solution containing 50 g/l of bismuth was prepared by colddissolution of 7.54 g of BiCl₃ under stirring in a beaker containing 60ml of 10% wt. HC1. Upon completion of the dissolution, once a clearsolution was obtained, the volume was brought to 100 ml with 10% wt.HCl.

10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solutionand 7.44 ml of the 50 g/l Bi solution were added to a second beaker keptunder stirring. The stirring was protracted for 5 more minutes. 10 ml of10% wt. acetic acid were then added.

Part of the solution was applied by brushing in 7 coats to thepreviously treated titanium sheet, carrying out a drying step at 60° C.for 15 minutes after each coat and a subsequent decomposition at hightemperature for 15 minutes. The high temperature decomposition step wascarried out at 480° C. after the first coat, at 500° C. after the secondcoat, at 520° C. after the subsequent coats.

In this way, a catalytic layer having an lr:Sn:Bi molar ratio of 33:61:6and a specific Ir loading of about 10 g/m² was applied.

The application of the external layer was then carried out (for anamount of 12 g/m² referred to the oxides) by brushing in 8 coats of anaqueous TaCI₅ solution, acidified with HCl. Three samples of 1 cm² areawere cut out from the electrode thus obtained and subjected to anaccelerated duration test under anodic oxygen evolution, by measuringthe deactivation time (defined as the time of operation required forobserving a 1 V potential increase) in H₂SO₄ at 150 g/l, at atemperature of 60° C. and at a current density of 30 kA/m². The averagedeactivation time of the three samples was found to be 600 hours.

An anodic potential of 1.556 V/NHE was measured at 1000 A/m².

EXAMPLE 2

A titanium sheet grade 1 of 200 m×200 m×3 mm size was pre-treated andprovided with a protective layer based on titanium and tantalum oxidesat a 80:20 molar ratio as in the previous example. A precursor solutioncontaining 50 g/l of tantalum was prepared by placing 10 g of TaCI₅ in abeaker containing 60 ml of 37% by weight HCl bringing the whole mixtureto boiling for 15 minutes under stirring. 50 ml of demineralised H₂Owere then added and the solution was kept under heating for about 2hours until the volume was back to 50±3 ml. 60 ml of 37% by weight HClwere then added obtaining a clear solution, again brought to boilinguntil the volume was back to 50 ±3 ml. The volume was then brought to100 ml with demineralised H₂O. To a second beaker kept under stirring,10.15 ml of the 1.65 M SnHAC solution of the previous example, 10 ml ofthe 0.9 M IrHAC solution of the previous example and 7.44 ml of the 50g/l Ta solution were added. The stirring was protracted for 5 minutes.10 ml of 10% by weight acetic acid were then added. Part of the solutionwas applied by brushing in 8 coats to the previously treated titaniumsheet, carrying out a drying step at 60° C. for 15 minutes after eachcoat and a subsequent decomposition at high temperature for 15 minutes.The high temperature decomposition step was carried out at 480° C. afterthe first coat, at 500° C. after the second coat, at 520° C. after thesubsequent coats.

In this way, a catalytic layer having an lr:Sn:Ta molar ratio of32.5:60:7.5 and a specific Ir loading of about 10 g/m² was applied.

The application of the external layer was then carried out (for anamount of 15 g/m² referred to the oxides) by brushing in 10 coats of anaqueous TaCI₅ solution, acidified with HCl. Three samples of 1 cm² areawere cut out from the electrode thus obtained and subjected to anaccelerated duration test under anodic oxygen evolution, by measuringthe deactivation time (defined as the time of operation required forobserving a 1 V potential increase) in H₂SO₄ at 150 g/l, at atemperature of 60° C. and at a current density of 30 kA/m². The averagedeactivation time of the three samples was found to be 520 hours.

An anodic potential of 1.579 V/NHE was measured at 1000 A/m².

COUNTEREXAMPLE 1

A titanium sheet grade 1 of 200 m×200 m×3 mm size was degreased andsubjected first to sandblasting with corundum grit until obtaining avalue of superficial roughness Rz of 70 to 100 μm, then to an etching in20% by weight HCl at a temperature of 90-100° C. for 20 minutes.

After drying, a protective layer based on titanium and tantalum oxidesat a 80:20 molar ratio was applied to the sheet, with an overall loadingof 0.6 g/m² referred to the metals (equivalent to 0.87 g/m² referred tothe oxides). The application of the protective layer was carried out bypainting in three coats of a precursor solution—obtained by addition ofan aqueous TaCI₅ solution, acidified with HCl, to an aqueous solution ofTiCI₄—and subsequent thermal decomposition at 500° C.

On the protective layer, a catalytic coating based on oxides of iridiumand tantalum at a 65:35 weight ratio (equivalent to a molar ratio ofabout 66.3:36.7) was then applied, with an overall iridium loading of 10g/m². The electrode was heat-treated at 515° C. for 2 h, then theapplication of the external layer was carried out (for an amount of 15g/m² referred to the oxides) by brushing in 10 coats of an aqueous TaCI₅solution, acidified with HCl. Three samples of 1 cm² area were cut outfrom the electrode thus obtained and subjected to an acceleratedduration test under anodic oxygen evolution, by measuring thedeactivation time (defined as the time of operation required forobserving a 1 V potential increase) in H₂SO₄ at 150 g/l, at atemperature of 60° C. and at a current density of 30 kA/m². The averagedeactivation time of the three samples was found to be 525 hours.

An anodic potential of 1.601 V/NHE was measured at 1000 A/m².

COUNTEREXAMPLE 2

A titanium sheet grade 1 of 200 m×200 m×3 mm size was degreased andsubjected first to sandblasting with corundum grit until obtaining avalue of superficial roughness Rz of 70 to 100 μm, then to an etching in20% by weight HCl at a temperature of 90-100° C. for 20 minutes. Afterdrying, a protective layer based on titanium and tantalum oxides at a80:20 molar ratio was applied to the sheet, with an overall loading of0.6 g/m² referred to the metals (equivalent to 0.87 g/m² referred to theoxides). The application of the protective layer was carried out bypainting in three coats of a precursor solution—obtained by addition ofan aqueous TaCI₅ solution, acidified with HCl, to an aqueous solution ofTiCI₄—and subsequent thermal decomposition at 500° C.

On the protective layer, a catalytic coating consisting of two distinctlayers was then applied: a first layer (internal) based on oxides ofiridium and tantalum at a 65:35 weight ratio (equivalent to a molarratio of about 66.3:36.7), with an overall iridium loading of 2 g/m² anda second layer (external) based on oxides of iridium, tantalum andtitanium at a 78:20:2 weight ratio (corresponding to a molar ratio ofabout 80.1:19.4:0.5), for an overall iridium loading of 10 g/m².

The application of the external layer was then carried out (for anamount of 15 g/m² referred to the oxides) by brushing in 10 coats of anaqueous TaCI₅ solution, acidified with HCl. Three samples of 1 cm² areawere cut out from the electrode thus obtained and subjected to anaccelerated duration test under anodic oxygen evolution, by measuringthe deactivation time (defined as the time of operation required forobserving a 1 V potential increase) in H₂SO₄ at 150 g/l, at atemperature of 60° C. and at a current density of 30 kA/m². The averagedeactivation time of the three samples was found to be 580 hours.

An anodic potential of 1.602 V/NHE was measured at 1000 A/m². Theprevious description shall not be intended as limiting the invention,which may be used according to different embodiments without departingfrom the scopes thereof, and whose extent is solely defined by theappended claims.

Throughout the description and claims of the present application, theterm “comprise” and variations thereof such as “comprising” and“comprises” are not intended to exclude the presence of other elements,components or additional process steps.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention before the priority date of each claim of thisapplication.

1. A method for manufacturing an electrode suitable for oxygen evolutionin electrolytic processes comprising: applying a solution containingprecursors of iridium, tin and doping element bismuth to a valve metalsubstrate and subsequently decomposing the solution by a thermaltreatment in air at a temperature of 480 to 530° C.; forming an externallayer by application and subsequent thermal decomposition of a solutioncontaining a precursor of tantalum pentoxide, thereby obtaining theelectrode suitable for oxygen evolution in electrolytic processescomprising: the valve metal substrate, the catalytic layer comprisingmixed oxides of iridium, of tin and doping element bismuth, the molarratio of Ir:(Ir+Sn) ranging from 0.25 to 0.55 and the molar ratio ofBi:(Ir+Sn+Bi) ranging from 0.02 to 0.15, a protective layer consistingof valve metal oxides interposed between the substrate and the catalyticlayer, and the external layer comprising tantalum pentoxide, wherein thespecific loading of tantalum pentoxide in the external layer ranges from12 to 15 g/m² referred to the oxides.
 2. The method according to claim1, wherein the protective layer is applied to the valve metal substrateprior to applying a solution containing precursors of iridium, tin anddoping element bismuth.
 3. The method according to claim 2, whereinafter applying the solution, a thermal decomposition is carried out. 4.The method according to claim 1, wherein the molar ratio Bi:(Ir+Sn+Bi)ranges from 0.05 to 0.12.
 5. The method according to claim 1, whereinthe molar ratio Ir:(Ir+Sn) ranges from 0.40 to 0.50.
 6. The methodaccording to claim 1, wherein the mixed oxides of iridium, of tin anddoping element bismuth in the catalytic layer consist of crystallites ofaverage size below 5 nm.
 7. The method according to claim 1, wherein theprotective layer consists of titanium and tantalum oxides.
 8. The methodaccording to claim 6, wherein the protective layer consists of a 80:20molar ratio of titanium and tantalum oxides.